This invention relates generally to the joining of materials, and more particularly, to a liquid phase bonding process for joining two or more pieces, at least one of which is an amorphous material.
Metals ordinarily solidify from the molten state as crystals having a periodically repeating crystalline structure. When properly processed, however, many normally crystalline materials may be prepared in an amorphous state exhibiting little or no structural periodicity. In some instances, such amorphous materials are observed to have extremely high strengths with sufficient fracture toughness to render them attractive engineering materials. Further, the amorphous materials have no grains or grain boundaries, and consequently are more resistant to attack by corrosion than are crystalline materials of the same composition. Amorphous materials are therefore important candidates for use as engineering structural materials.
The amorphous materials known to date are typically prepared by rapid solidification from the liquid state at cooling rates of about 10.sup.5 .degree. C. per second, or greater. To achieve the necessary high cooling rates, the amorphous materials are solidified as thin sheets or strips having a thickness of less than about 0.07 mm by depositing a liquid alloy on a cool substrate as a thin layer so that heat is extracted very rapidly and high cooling rates are achieved. Alternatively, high cooling rates may be achieved by solidifying the amorphous material as a powder directly from the liquid state. Other procedures for preparing amorphous materials are known or under development, but generally, the preparation techniques limit the form of the as-prepared amorphous material to thin strips or powders.
Although thin strips or powders of materials have some limited engineering uses, most applications require that these basic forms be assembled into larger pieces that are more readily utilized. For example, a thin strip may be used as an overlay to protect a substrate, but this use requires that the thin strip be attached to the substrate in some fashion. In another example, powders are typically sintered into parts for subsequent use.
The ability to join or bond amorphous materials by conventional metallurgical techniques is limited by the crystallization of the amorphous material. Amorphous materials may be converted to the crystalline state by introducing sufficient energy to induce a transformation to a periodic structure, as by heating the amorphous material above a "crystallization temperature" which may be readily determined for each material by conventional techniques. Many of the beneficial properties of the amorphous state are lost upon crystallization, and it is therefore necessary that a bonding or joining technique avoid inducing crystallization. Thus, there exists a need for a bonding or joining technique for fabricating the as-prepared amorphous materials into engineering structures.
Certain conventional bonding or joining techniques may be utilized in specific applications, but in others suffer from serious drawbacks. Conventional fasteners such as bolts may be used to join amorphous pieces such as strips, but these fasteners introduce undesirable stress concentrations, may be highly susceptible to corrosive attack in media wherein the corrosion-resistant amorphous material may otherwise be used, and cannot be used in some applications due to physical limitations. Moreover, fasteners do not produce a continuous bond across the joined surfaces of the pieces.
Conventional adhesives such as glues or epoxies may be used to join amorphous pieces with a continuous bond, but the nonmetallic bonding agent rapidly loses strength at increasing temperatures and also is susceptible to degradation by wear. Many of the projected applications of amorphous materials require a fully metallic structure, including the bonding means.
Brazing and soldering techniques for joining metals produce a fully metallic final structure. However, with both of these techniques the final bonded structure includes an intermediate layer of crystalline material. If the bonded part is to be used in a corrosive environment, this intermediate layer will most likely be preferentially attacked by the medium, leaving an incipient crack at the interface between the bonded pieces. Further, the alloying ingredients in the brazing or soldering material may diffuse into the parts to reduce the crystallization temperature of the amorphous material by an alloying effect. Such an alloying effect leads to an undesirable reduction in the potential operating temperature of the final bonded part.
Powdered amorphous materials cannot be joined using conventional sintering techniques such as hot pressing, as the required high temperature induces crystallization of the powder particles.
While a number of techniques for bonding and joining amorphous materials have been proposed or may be visualized from conventional joining techniques, no technique has been proposed for joining amorphous materials to form a continuously bonded, fully metallic, amorphous structure. Accordingly, there exists a need for such an approach. The present invention fulfills this need, and further provides related advantages.